This work evaluates the changes in radiative properties of two decomposing carbonaceous porous materials, each composed of two semitransparent, homogeneous and isotropic phases. The understanding of the complex dependence of macroscopic optical behaviour on material microstructure, bulk phase properties and the wavelength of incoming radiation is paramount for modelling, design and optimisation of systems incorporating such media. Experimental and numerical techniques were combined to solve the homogenised radiative transfer equations using Monte Carlo ray tracing in the limit of geometrical optics. Effective radiative properties required by these equations were determined by Monte Carlo techniques using the exact 3D microstructures of the samples, obtained through high-resolution synchrotron computed tomography. This methodology is applied to medium density carbon phenolic and high density graphite reinforced polymer composite, each composed of semi-transparent solid and fluid phases. The extent of material decomposition is seen to affect the absorption behaviour of both samples. This effect is more obvious in the lower density carbon phenolic, where an 18% increase in absorptance is observed due to decomposition, compared to an increase of just 2% for the graphite. A library of absorption data is presented for use in continuum heat transfer modelling of similar chemically reacting macroporous carbon composites.